In general, di/dt based current measurement devices are used for measuring very low alternating current in a conductor, conductors which may also intermittently be subject to extremely high peak transient currents. Existing di/dt based current measurement sensors have inherent limitations, which inhibit their use in this type of application.
In a di/dt sensor, a voltage proportional to the amplitude as well as the frequency of the current being measured is generated in the sensor.
One example of a di/dt sensor is the planar magnetic printed circuit board current sensor described in Irish Patent Application No. S2001/0370.
As shown in FIGS. 1(a) and 6, Irish Patent Application No. S2001/0370 describes a current sensor 10 comprising a number of inductive PCB sensor elements, or coil boards 12 which are vertically mounted in a symmetric fashion around a current-carrying conductor (not shown) on a motherboard 14. The coil boards 12 carry a pair of substantially identical coils C1-C14 of non-magnetic material, which are arranged in a notional loop as shown in FIG. 6. The current sensor also comprises means 115 for deriving the alternating current in the conductor as a function of the voltages induced in the series-connected coils C1-14.
In the known PMCS sensor 10, as shown in FIGS. 1a and 6, the input noise to the input amplifier 115 of the signal conditioning stage is typically in the region of 30 nV. In order for the sensor 10 to measure the 0.1 mA, the pickup voltage presented to the amplifier 115 must be at least 30 nV to give a signal to noise ratio of 1.
The sensor, however, may be used in an environment where it is subject to transient spikes of current, and this causes certain difficulties. For example, a typical sensor will be normally used to measure an alternating current in a conductor, of frequency 50 Hz, and of minimum amplitude 0.1 mA. At certain intervals, this same conductor might be subject to transient current pulses, with a rise time of 4 μS and an amplitude of greater than 100 kA, such as when lightning strikes occur.
Assuming the frequency of the 100 kA pulse current to be at 50 Hz, then the voltage pickup of the sensor 10 would be larger than 30 nV, by a factor of 100 kA divided by 0.1 mA, i.e. a factor of 109 greater. This would lead to a voltage induced in the di/dt sensor 10 of 30 nV multiplied by 109, equal to 30V.
However, as mentioned earlier, the amplitude of the induced voltage is not only dependent on the amplitude of the current, but also on the frequency, and the frequency of such transient currents will not normally be the mains frequency of 50/60 Hz.
So, for a current pulse with a rise time of 4 μS, the actual voltage developed in the sensor 10 would be equal to the voltage induced at 50 Hz multiplied by the ratio of the actual pulse frequency to the 50 Hz frequency, i.e.
  V  =            30      ⁢                          ⁢      nV      ×              10        9            ×              f        p              50  where fp is the equivalent frequency of the pulse.
The equivalent frequency of a pulse of rise time tr is given as
      f    p    =      1          2      ⁢                          ⁢      π      ⁢                          ⁢              t        r            
Therefore, the voltage induced in the sensor 10 by a pulse of rise time 4 μS (equivalent frequency of about 40 kHz) and an amplitude of 100 kA is as follows:
  V  =                    30        ⁢                                  ⁢        nV        ×                  10          9                            2        ⁢                                  ⁢        π        ×        4        ⁢                                  ⁢        µS        ×        50              =                            30          ×                      10                          -              9                                ×                      10            9                                    2          ×          3          ×          4          ×                      10                          -              6                                ×          50                    .      which gives a value of approx 25,000V if the value of π is approximated to 3.
This is a very large value of voltage for a practical current sensor to be able to handle. Any voltage tracks across which this value of voltage will appear will need to be spaced sufficiently to avoid flashover. For a voltage level of 25,000 volts such spacing would be in the some tens of centimeters. This would not only lead to increasing the size of the sensor to an impractically large size, but the large spacing between the tracks leading from the pickup coils would cause the sensor to be more susceptible to external interference, as described in the aforementioned patent.
As well as this, all components across which this voltage appears, (e.g. resistors, op-amps) would have to be sufficiently rated such as to be able to endure this voltage level without deterioration of performance or damage to the component.
It can be quickly deduced from the above that to manufacture such a sensor on a commercial basis is prohibitively expensive and impractical.
This invention describes improvements to current sensors which aim to minimise these limitations, thereby enabling a di/dt based current sensor to be manufactured at a reasonable cost, and which can operate satisfactorily in the environment described.